The present invention is directed to a structure for use in interlocking together two elements and, in particular, to a structure for joining together parts of a medical implant. The structure includes a first interlocking form on a closure and a mating second interlocking form on a receiver. The closure is operably rotated into the receiver. The first and second interlocking forms are both helically wound so that the first interlocking form advances relative to the second interlocking form, when the closure with the first interlocking form is inserted in the receiver and rotated. At least one of the first or second interlocking forms includes a projection that overlaps and radially locks with the other interlocking form when the two forms are mated.
Medical implants present a number of problems to both surgeons installing implants and to engineers designing them. It is always desirable to have an implant that is strong and unlikely to fail or break during usage. It is also desirable for the implant to be as small and lightweight as possible so that it is less intrusive on the patient. These are normally conflicting goals, and often difficult to resolve.
One particular type of implant presents special problems. In particular, spinal bone screws, hooks, etc. are used in many types of back surgery for repair of injury, disease or congenital defect. For example, spinal bone screws of this type are designed to have one end that inserts threadably into a vertebra and a head at an opposite end thereof. The head is designed to receive a rod or rod-like member in a channel in the head which rod is then both captured in the channel and locked in the head to prevent relative movement between the various elements subsequent to installation.
There are two different major types of bone screws and similar devices which are classified as closed headed and open headed. While the closed headed devices are highly effective at capturing and securing a rod, since the rod is threaded through an opening in the head, it is very difficult during surgery to thread the rod through the heads. This is because there are many heads and the rod is curved or the heads do not align. Consequently, the more screw heads that the rod must pass through, the more difficult it is to thread the rod into them.
The second type of head is an open head wherein a channel is formed in the head and the rod is simply laid in an open channel. The channel is then closed with a closure member. The open headed bone screws and related devices are much easier to use and in some situations must be used instead of the closed headed devices.
While the open headed devices are often necessary and often preferred for usage, there is a significant problem associated with them. In particular, the open headed devices conventionally have two upstanding arms that are on opposite sides of a channel that receives the rod member. The top of the channel is closed by a closure member after the rod member is placed in the channel. The closure can be of a slide in type, but such are not easy to use. Threaded nuts are sometimes used that go around the outside of the arms. Such nuts prevent splaying of the arms, but nuts substantially increase the size and profile of the implant which is not desirable. Many open headed implants are closed by plugs that screw into threads between the arms, because such have a low profile. However, threaded plugs have encountered problems also in that they produce radially outward forces that lead to splaying of the arms or at least do not prevent splaying that in turn loosens the implant. In particular, in order to lock the rod member in place, a significant force must be exerted on the relatively small plug or screw. The forces are required to provide enough torque to insure that the rod member is clamped or locked in place relative to the bone screw, so that the rod does not move axially or rotationally therein. This typically requires torques on the order of 100 inch-pounds.
Because open headed implants such as bone screws, hooks and the like are relatively small, the arms that extend upwardly at the head can be easily bent by radially outward directed forces due to the application of substantial forces required to lock the rod member. Historically, early closures were simple plugs that were threaded with V-shaped threads and which screwed into mating threads on the inside of each of the arms. But, as noted above, conventionally V-shaped threaded plugs tend to splay or push the arms radially outward upon the application of a significant amount of torque, which ends up bending the arms sufficiently to allow the threads to loosen or disengage and the closure to fail. To counter this, various engineering techniques were applied to allow the head to resist the spreading force. For example, the arms were significantly strengthened by increasing the width of the arms by many times. This had the unfortunate effect of substantially increasing the weight and the profile of the implant, which was undesirable.
Many prior art devices have also attempted to provide outside rings or some other type of structure that goes about the outside of the arms to better hold the arms in place while the center plug is installed and thereafter. This additional structure may cause the locking strength of the plug against the rod to be reduced which is undesirable, especially when the additional structure is partly located beneath the plug. Also, the additional elements are unfavorable from a point of view of implants, since it is typically desirable to maintain the number of parts associated with the implants at a minimum and, as noted above, the profile as minimal as possible.
Other designers have attempted to resolve the splaying problem by providing a closure with a pair of opposed radially extending wedges or flanges that have mating structure in the arms of the implant. Such devices serve as a closure and do somewhat resist splaying of the arms, but are often very difficult to use. In particular, the rods normally have some curvature as the rods are bent to follow the curvature of the spine and normally bow relative to the bottom of the bone screw channel that receives such a rod. The rod thus fills much of the channel and must be “unbent” to rest on the bottom of the channel and be held securely in place. Therefore, the rod is preferably compressed by the plug and unbent by advancement of the plug into the channel in order to assume that the plug will securely hold the rod and that the rod and plug will not loosen when post assembly forces are placed on the rod. Because it takes substantial force to unbend the rod, it is difficult to both place the plug fully in the channel and rotate it for locking while also trying to line up the wedges with the mating structure. It is much easier to align the plug mating structure with the mating structure of the arms at the top of the arms and then rotate the plug so as to screw the plug into a plug receiver to advance the plug toward the rod. In this way the plug starts applying significant force against the rod only after parts of the mating structure have at least partly joined at which time torque can be applied without having to worry about alignment. It is noted that where wedges are used, the cross section of the structure changes therealong so that the device “locks up” and cannot turn further after only a small amount of turning, normally ninety degrees.
Consequently, a lightweight and low profile closure plug was desired that resists splaying or spreading of the arms while not requiring significant increases in the size of the screw or plug heads and not requiring additional elements that encircle the arms to hold the arms in place.
It is noted that the tendency of the open headed bone screw to splay is a result of the geometry or contour of the threads typically employed in such devices. In the past, most bone screw head receptacles and screw plugs have employed V-shaped threads. V-threads have leading and trailing sides oriented at angles to the screw axis. Thus, torque on the plug is translated to the bone screw head at least partially in an axial direction, tending to push or splay the arms of the bone screw head outward in a radial direction. This in turn spreads the internally threaded receptacle away from the thread axis so as to loosen the plug in the receptacle.
The radial expansion problem of V-threads has been recognized in various types of threaded joints. To overcome this problem, so-called “buttress” threadforms were developed. In a buttress thread, the trailing or thrust surface is oriented perpendicular to the thread axis, while the leading or clearance surface remains angled. This theoretically results in a neutral radial reaction of a threaded receptacle to torque on the threaded member received.
Development of threadforms proceeded from buttress threadforms which in theory have a neutral radial effect on the screw receptacle to reverse angled threadforms which theoretically positively draw the threads of the receptacle radially inward toward the thread axis when the plug is torqued. In a reverse angle threadform, the trailing side of the external thread is angled toward the thread axis instead of away from the thread axis, as in conventional V-threads. While buttress and reverse threadforms reduce the tendency to splay, the arms can still be bent outward by forces acting on the implant and the threads can be bent by forces exerted during installation. Therefore, while certain threadforms may not exert radial forces during installation, at most such threadforms provide an interference or frictional fit and do not positively lock the arms in place relative to the closure plug.
Finally, it is noted that plugs of this type that use threadforms are often cross threaded. That is, as the surgeon tries to start the threaded plug into the threaded receiver, the thread on the plug is inadvertently started in the wrong turn or pass of the thread on the receiver. This problem especially occurs because the parts are very small and hard to handle. When cross threading occurs, the plug will often screw part way in the receiver and then “lock up” so that the surgeon is led to believe that the plug is properly set. However, the rod is not tight and the implant fails to function properly. Therefore, it is also desirable to have a closure that resists crossthreading in the receiver.